In many electronic applications, electrical resonators are required. For example, in many wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as voltage controlled oscillators (VCOs). As is known, VCOs are components of phase locked loops (PLLs) and frequency synthesizers used in wireless transceivers, such as mobile telephones.
One type of resonator is a high tone bulk acoustic resonator (HBAR). The HBAR comprises a piezoelectric transducer disposed over a support substrate. In general, the piezoelectric transducer includes a piezoelectric layer disposed between metal electrode layers. The substrate of the HBAR has a thickness that is significantly greater than that of the piezoelectric layer of the transducer. As a result, the HBAR exhibits multiple resonances that are equally spaced, with the spacing being determined by the thickness of the substrate.
The quality (Q) factor of the resonances in the HBAR is proportional to the ratio of the stored energy (longitudinal standing wave) to the lost energy per cycle. Because the thickness of the support substrate of the HBAR is greater than the thickness of the piezoelectric and metal layers of the transducer, most of the acoustic energy of the HBAR is stored in the substrate. Thus, the Q-factor of the HBAR is dominated by acoustic loss (e.g., attenuation) in the substrate.
As will be appreciated from the above description, it is beneficial to include a comparatively low acoustic loss material as the substrate of the HBAR. Unfortunately, many of the materials that exhibit low acoustic loss are often difficult to process in large scale component or circuit fabrication. Moreover, regardless of the material chosen for the substrate, certain processing steps can be deleterious to the HBAR devices and other components during manufacture. In addition, other factors beside acoustic attenuation in the substrate can adversely impact the Q-factor of the HBAR.
What is needed, therefore, is an oscillator and method of fabrication that address at least the shortcomings described above.